1. Field of the Invention
The present invention relates to a method and apparatus for heating a sheet-like material to a predetermined temperature profile. Such a method is used, for example, in annealing processes prior to forming sheets and plates of metal materials, as well as in furnaces for continuous heat treatment of sheet metals.
2. Description of the Related Art
When heat treating sheets, plates, etc., of a metal material such as steel, it is often desired to be able to control the material characteristics across the heat treated material.
The characteristics can include, by way of example, material hardness, flatness, and residual stress.
An example of such a heat treatment process is when annealing sheets of metal in a furnace prior to forming. In that case, material characteristics that are uniform across the metal sheet are often desired, both in the longitudinal as well as in the transverse directions with respect to the direction of material flow in the heat treatment process, because that provides good formability behavior of the metal sheet in many applications. In order to obtain such uniform material characteristics, it is necessary for the heat transfer to the metal sheet to be uniform across the sheet, in order to obtain a uniform temperature distribution or temperature profile across the entire sheet.
In other applications, a non-uniform, predetermined temperature profile is desired. For example, different hardness characteristics can be wanted on the edges of a metal sheet than at its center, for further processing into a product such as a car roof or the like.
Today, the heat treatment of sheet-like metals usually takes place in a furnace. Commonly used furnaces include fuel-based furnaces that can have an open flame or a heating tube for transferring heat to the metal sheet.
When using such furnaces for the heat treatment of, for example, a metal sheet, it is often not possible to obtain the desired temperature profile across the sheet. Instead, a number of problems occur.
Firstly, prior art furnaces for heat treatment of sheet-like metal materials experience problems with overheated edges, as compared to the heating of the mid-sections of the sheets. The reason for that is that toward the edge of the sheet, the surface area/volume ratio of the sheet increases, which gives rise to faster heat transfer into the metal at the edges. That is common when heat treating sheet or plate products with thicknesses ranging from 1 mm to 100 mm, but is also an issue for materials with an even larger thickness (for example up to 300 mm), and across the whole range of metal materials, including carbon steel, stainless steel, mild steels, aluminum, copper, etc. The temperature difference between the edge and the center of the sheet can be as much as 20° C.
In the case when heat treating metal sheets one by one, the problem arises both at the side edges of the sheet, as well as at the starting and the ending edges. For continuous processing of a long metal sheet, the problem arises mainly at the side edges, but possibly also when starting or stopping the process, or when changing sheets.
The result of that problem is that the transverse and longitudinal temperature differences lead to deformations, uneven hardness, and/or other material characteristics that are non-uniformly distributed across the sheet. In some cases, sheets have to be straightened prior to the next processing step, further deteriorating the hardness and residual stress characteristics of the material. Of course, the problem occurs both in the longitudinal as well as in the transverse direction across the sheet.
Secondly, it is difficult to precisely control the temperature profile, in any direction, across sheet-like metals when using conventional furnaces. As described above, a specific, non-uniform temperature profile might be desired in order to render the heat treated metal suitable for further processing in various applications. Control over the temperature profile is often desired both in the longitudinal and in the transverse directions of the sheet.
Thirdly, in some applications it is desired that some sections of the sheet-like metal are heat treated at different times from other sections. For example, when annealing a metal sheet, the inventors have shown it to be advantageous to heat the mid-section of the sheet first, in order to introduce compressive stress in the mid-section. Thereafter, it is advantageous to transfer heat to the edge of the sheet. That way, the compressive stress introduced in the edges of the sheet will not cause the sheet to deform when the sheet is annealed. That will be described in greater detail below.
The present invention solves the above-described problems.